20 THE BONES. 



portion of the nervous centres the spinal marrow. This canal, which 

 traverses the vertebra from one end to the other, transforms it into a 

 veritable ring in which we recognise, for facility of description, two parts 

 the one inferior, the other superior. The first, or body, is very thick, and 

 forms the base of the vertebra ; the second, which is thin, has been de- 

 signated spinous or spinal, from one of the peculiarities it presents, or annular, 

 because it circumscribes the major portion of the spinal foramen. This 

 division is not altogether an arbitrary one, for the body and the annular por- 

 tion constitute, in the foetus, two distinct pieces, which do not become 

 united for a long time after birth. 



BODY. The shape of the body of a vertebra is that of a prism with four 

 faces, of which two only the superior and inferior are free, and can be 

 studied in the adult ; the two lateral faces being united and confounded 

 with the annular portion. This prism also presents two extremities an 

 anterior and posterior. 



Faces, The superior face, limited in extent, forms part of the spinal 

 foramen, constituting its floor. It exhibits: 1, On the median line, two 

 roughened, prominent surfaces, representing two triangles, whose summits 

 are opposed ; 2, On the sides, two depressed smooth surfaces, perforated by 

 one or more openings that lead to the interior of the bone. The inferior 

 face is divided into two lateral portions by a median crest. 



Extremities. The anterior has a prominent convex head, more or less 

 detached. The posterior offers a cavity for the reception of the head of the 

 next vertebra. These two planes, the one convex, the other concave, do not 

 come into immediate contact ; an elastic, flexible fibro -cartilage, firmly 

 attached to each, being interposed between them. 



ANNULAR PORTION. This is formed by an osseous plate that curves sud- 

 denly downwards, in the shape of an arch, the two extremities of which 

 approach each other, inclose the body, and become united to it. It offers 

 for study : 1, An internal and an external surface ; 2, An anterior and a 

 posterior border. 



Surfaces. The internal surface, concave and smooth, forms, with the 

 superior face of the body, the spinal foramen. The external, convex and 

 irregular, presents : 1, A single prominence, raised in the middle of the 

 superior portion, and named the spinous process ; 2, The transverse processes 

 are a double pair of eminences, one on each side, and projected transversely 

 outwards. 



Borders. The anterior border has two articular facets looking upwards : 

 these are the anterior articular processes, right and left. In each is a notch 

 which, when placed in opposition to a similar excavation in the preceding 

 vertebra, forms the intervertebral foramen. The posterior border presents the 

 same peculiarities, with this difference, that the articular faces of the pos- 

 terior articular processes are inclined downwards, to correspond with the 

 anterior facets of the succeeding vertebra. 



Structure of the vertebrae. The compact substance, which is abundant in 

 the spinous portion, forms, in the body, an extremely thin layer, inclosing 

 a voluminous nucleus of spongy tissue. The latter is traversed by numerous 

 venous canals, which open on the surface of the bone. 



Development. It has been already shown that the body and spinous 

 portion of a vertebra constitute, in young animals, two distinct pieces. Each 

 was primarily formed from two lateral centres, which met on the median line. 

 In the body, the fusion of these centres is so prompt, that it is generally 

 believed, perhaps justly, that the development of this part of the vertebra 



